The invention relates to a power system and to a working machine comprising such a power system.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders, articulated haulers and excavators. Although the invention will be described hereinafter with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other heavy working machines, such as dump trucks, or other construction equipment.
A working machine is typically provided with an internal combustion engine for propulsion of the working machine via a gear box, and for providing power w one or more auxiliary hydraulic systems of the working machine. Such hydraulic systems can include steering and one or more working hydraulic circuits of a working machine.
One or more hydraulic pumps can be used to provide hydraulic power to the one or more hydraulic systems of the working machine, and the pumps may typically be powered by the internal combustion engine of the working machine through a power take off (PTO) connected to a main shaft of the engine.
The power system of a working machine must be able to provide power to at least two different mechanical loads, where propulsion is an example of a rotary load, and a work hydraulic system is an example of a linear load when it is provided in the form in the form of one or more linear hydraulic actuators, i.e. hydraulic cylinders powering a lifting motion of an arm or a bucket. In other cases the work hydraulics could instead contain rotary loads, i.e. hydraulic motors powering a swing function of an excavator or a saw motor.
Furthermore, there is an increasing interest in improving the energy efficiency of working machines as the operational cost of a working machine is related to the fuel consumption and as environmental concerns limits the amount of contaminants that may be released by the working machine.
Taking a wheel loader as an example, the hydraulics system is connected to a PTO on the drive shaft of the engine and so is the input axle to the transmission of the propulsion system. The transmission system may for instance be based on a hydrodynamic torque converter which torque output depends on the differential speed between its incoming and outgoing axles. The pumps of the hydraulic system could have fixed or variable displacement.
With this system architecture the rpm of the engine determines the rpm of the hydraulic machine, but at the same time it also determines the speed of the input axle to the propulsion system. This means that the maximum flow of the hydraulic machine is limited by the engine speed and so is the transmission torque. In some cases high hydraulic flow is needed, but the torque request on the propulsion is low, a high engine speed is needed to satisfy the hydraulic system, which will lead to big power losses over the torque converter since the operator will stop the machine from going forward by applying the friction brakes, and heat will be generated over the torque converter. Also the other way around can be a problem; the propulsion system requires a high engine rpm to perform the commanded torque output, while the hydraulics only has a very small flow demand. This situation will also cause problems, but this time power losses occur in the hydraulic system, since the hydraulic machine must operate at partial displacement, where the efficiency is lower compared to when operating, at full flow.
Other issues in the motion systems of most construction machines today is that kinetic energy stored in the mass of a moving machine is not recovered when the machine is decelerated, nor is the potential energy of an elevated load recovered when the load is lowered. Today this kinetic and potential energy is generally wasted as heat over friction brakes and hydraulic resistances. To solve these problems, the power system needs to be complemented with some sort of energy recovery system and in case energy should be stored over time also an energy storage system is needed.
Furthermore, it is desirable to increase the efficiency of the components in the hydraulic circuit including the power take-off, where hydraulic mechanical energy is converted into hydraulic, energy.
A proposed approach for increasing the energy efficiency in a power system is to provide a hydraulic system for the work hydraulics of a working machine where a hydraulic accumulator is connected to the same hydraulic node as the hydraulic machine of the power take-off for providing hydraulic energy to the work hydraulics. Thereby, excess hydraulic energy can be stored in the accumulator. For example, energy can be recuperated from the work hydraulic system. This may facilitate a reduction in size of the hydraulic machine, which in turn leads to increased energy efficiency.
However, even though more energy efficient components and systems are developed, there is still a need for a more energy efficient power system for a working machine.
It is desirable to provide a power system for a working machine energy may be recuperated m a more efficient manner using a power system with reduced complexity compared to prior art solutions.
According to a first aspect of the present invention, it is therefore provided a power system for a working machine. The power system comprises a transmission for driving the working machine, which transmission comprises a gear box having a gear unit and a hydraulic variator unit.
Furthermore, the power system comprises a work hydraulic circuit for controlling at least one hydraulic actuator of the working machine, and the power system is characterized in that the hydraulic variator unit is hydraulically connected to the work hydraulic circuit to hydraulically transfer energy from the hydraulic variator unit to the work hydraulic circuit.
The transmission is configured to be connected to a prime mover of the working machine, such as an internal combustion engine, for providing propulsion of the working machine. The continuously variable gear box comprises a hydraulic variator unit system which is operatively coupled to the mechanical gear unit by at least a first planetary gear train. Thereby, energy may be transferred from the engine to an output shaft of the gear box both via a mechanic path as well as via a hydraulic path. That the gear box is a continuously variable gearbox gives a plurality of advantages in relation to a stepped gearbox. For example, the velocity of the working machine can be controlled independently of the speed of rotation of the prime mover.
Furthermore, if the continuously variable transmission has a speed of rotation range comprising a mode where the rotation speed of the output shaft of the gearbox is zero or close to zero independently of the rotation speed of the input shaft of the gearbox at the same time as torque can be transmitted from the input shaft to the output shaft (usually referred to as an infinitely variable transmission (IVT) with geared neutral), the torque converter traditionally used in some working machines can be omitted.
A variable gear box of the type referred to herein, where power is split up into a mechanical patch and a hydraulic path, may also be referred to as a power-split continuously variable transmission (PS-CVT) gear box.
The work hydraulic circuit may for example comprise one or more hydraulic actuators for providing functions of the working machine such as lifting a load or steering.
The present invention is based on the realization that the hydraulic variator unit of a transmission in a working machine may be used also for providing hydraulic energy to a work hydraulic circuit of the working machine. Accordingly, there is no need for a separate power take-off and the hydraulic machine normally used for providing power to the work hydraulics is no longer required. Thereby, a power system is provided requiring fewer components, and which system can thus be made at a lower cost.
Furthermore, a continuously variable transmission is advantageously used in a power system according to the present invention as separation of operation of a hydraulic system from the propulsion of the working machine is facilitated, since the hydraulic system can be powered by an input shaft, which may be kept running, while an output shaft connected to propulsion drive train can be kept at stand still.
Accordingly, a pressure level required by the work hydraulic circuit can be provided by controlling the prime mover and the hydraulic variator unit.
According, to one embodiment of the invention, the power system may further comprise a hydraulic energy storage hydraulically connected to the hydraulic variator unit. Furthermore, the power system may be further configured to hydraulically transfer energy from the variator unit to the hydraulic energy storage. The hydraulic energy storage may for example be a hydraulic gas accumulator.
Moreover, the power system may further be configured to hydraulically transfer energy from the hydraulic energy storage to the work hydraulic circuit. Accordingly, energy transfer from the variator unit to the work hydraulic circuit may take place via the hydraulic energy storage. Whether the energy transfer from the variator unit to the work hydraulic circuit is taking place via the hydraulic energy storage or not may be controlled by valve means suitably arranged in the power system.
It is further realized that it is advantageous to share a hydraulic energy storage between the hydraulics based transmission and the work hydraulics of a working machine, where a common hydraulic machine is used for both the transmission and the work hydraulics. By using a solution comprising an accumulator for storing and releasing energy, for providing power to the work hydraulic circuit, it is possible to reduce the size of the overall power supply system, i.e. of the engine and of hydraulic machines of the variator unit.
Reducing the size of the engine and/or of the hydraulic, machine again lead to a power system which can be made at a lower cost, and to a working machine having an improved energy efficiency and thereby a reduced fuel consumption.
According to one embodiment of the invention, the power system may be configured to hydraulically transfer energy from the work hydraulic circuit to the hydraulic energy storage.
Furthermore, the power system may be configured to hydraulically transfer energy from the work hydraulic circuit to the hydraulic variator unit. Thus, further advantages can be found as energy may be recuperated both from the propulsion system and from the work hydraulics, and the recuperated energy may either be used or stored in the energy storage.
Accordingly, the power system according to various embodiments of the invention may in the end lead to both a substantial reduction in fuel consumption of the working machine and to a power system which comprises fewer parts.
In one embodiment of the invention, the hydraulic variator unit may comprise a first hydraulic machine and a second hydraulic machine, and at least one of the first hydraulic machine and the second hydraulic machine is hydraulically connected to the work hydraulic circuit. The chosen design of the mechanical gearbox connected to variator the hydraulic machines sets the requirements on which type of hydraulic machines that should be used inside the variator unit. However, to get the full advantage of the invention it is recommended that hydraulic machines are capable of reversed power flow transformation (in order to support recuperation of energy). This implies the use of a hydraulic machine that can operate in at least in two quadrants, meaning both as a pump and as a motor. The practical way to solve this is to adopt hydraulic machines that allow a reversed flow direction (through negative displacement capability). This type of machine is commonly referred to as “open circuit, over-center machines” which is commonly used in “secondary control system” applications. However the motoring capability can also be achieved with a machine only capable of unidirectional flow capability, but in this case the reversed power flow must be achieved through switching valves, changing side of pressurization of the hydraulic machines. For this solution another type of hydraulic machine is needed, since there is no longer any predetermined high and low pressure side. This type of machine is commonly referred to as a closed-circuit hydraulic machine.
According to one embodiment of the invention, at least one of the first hydraulic machine and the second hydraulic machine may be hydraulically connected to the hydraulic energy storage. In a conventional system, if the same hydraulic energy storage is used by the driveline and the work hydraulics, where the work hydraulics is powered by a power take-off and a separate hydraulic machine, there may be problems as the hydraulic machine of the work hydraulic must be of significant size to be able to be handle powers coming back from the propulsion system. According to the present embodiment, one or both of the hydraulic machines can be used to transfer power to both the driveline for propulsion and to the work hydraulics, thereby making the power limitation encountered in prior art solutions less of a problem. Furthermore, fewer power transformations are required when recuperating energy from the propulsion system compared to prior art solutions. Hence, higher recuperation efficiency is expected.
According to one embodiment of the invention, the power system ma further comprise a control unit configured to maintain a pressure level of the hydraulic energy storage within a first predetermined range. The control unit can he active to control the hydraulic flow in the system such that a pressure level of the hydraulic accumulator is maintained above a predetermined minimum value and below a predetermined maximum value. For example, if the pressure level of the hydraulic accumulator is approaching its minimum value, the engine can be activated to provide hydraulic flow via the variator unit to the accumulator so that the pressure level of the accumulator is increased. Furthermore, if the pressure level of the hydraulic accumulator is below its maximum value, and if the power system is in a mode where energy may be recuperated, either from the propulsion of from the work hydraulics, the control unit may advantageously redirect recuperated energy to the hydraulic accumulator. The control unit may be any unit suitable for controlling functions in a hydraulic system such as a microprocessor or the like configured to control for example the variator unit or a separate accumulator charging system to control pressure levels in one or more hydraulic accumulators.
In one embodiment of the invention, the power system may further comprise a second hydraulic energy storage hydraulically connected to the hydraulic variator unit and to the work hydraulic circuit. The second accumulator could also advantageously be connected to the low pressure side of the work hydraulic system. The main reason to use a low pressure accumulator is to maintain a sufficiently large pressure on the suction side of any reciprocating hydraulic element, such as a pump or cylinder in order to avoid cavitation. If a second accumulator is not used in the system, the normal solution is instead to use a hydraulic tank pressurized by atmospheric pressure.
Furthermore, the control unit may advantageously be configured to maintain a pressure level of the second hydraulic energy storage within a second predetermined range different from the first predetermined range. By providing hydraulic accumulators having different pressure levels, different components of the work hydraulic circuit requiring different hydraulic pressure may be powered by different hydraulic accumulators.
According to one embodiment of the invention, the variator unit may advantageously be input coupled. That the hydraulic machine is input coupled means that one hydraulic machine of the variator unit is coupled to the input shaft of the gear box.
According to one embodiment of the invention, the variator unit may advantageously be output coupled. That the hydraulic machine is output coupled means that one hydraulic machine of the variator unit is coupled to the output shaft of the gear box. However, the variator unit may advantageously also be connected in a variable bridge. The variable bridge is characterized by a four axle planetary gear-set to which input and output as well as two hydraulic machines are connected. The planetary gear-set consist of or comprises two planetary gears connected together. With this configuration many combinations exist to how two planetary gears can be connected together.
In one embodiment of the invention, the work hydraulic circuit may advantageously comprise a digital hydraulic actuator. A digital hydraulic actuator refers to an actuator comprising a cylinder having different effective areas, which are connected to the hydraulic circuit so that different discrete pressure levels may be provided to the different areas in order to provide different load levels of the actuator. Typically, discrete pressure levels are provided by hydraulic accumulators. As no throttling is required to achieve different load levels, the energy efficiency of a digital hydraulic actuator is improved in relation to a conventional hydraulic actuator. Furthermore, it is particularly advantageous to include a digital actuator in a power system according to various embodiments of the present inventions as the one or more accumulators already included in the power system also may be used for providing different discrete pressure levels to the digital actuator.
According to one embodiment of the invention the hydraulic variator unit may advantageously comprise a primary hydraulic machine and a secondary hydraulic machine, wherein at least one of the primary hydraulic machine and the secondary hydraulic machine is hydraulically connected to the work hydraulic circuit; and wherein at least one of the primary and secondary hydraulic machine comprises: a first rotating hydraulic machine and a second rotating hydraulic machine, the first and second hydraulic machine being arranged to provide a torque via a common output shaft; a first valve means for providing a differential hydraulic pressure level over the first hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels; a second valve means for providing a differential hydraulic pressure level over the second hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels; a control unit configured to control the first valve means and the second valve means such that different discrete levels of torque are provided via the output shaft.
Different discrete levels of torque can advantageously be provided by applying a differential pressure over two separate hydraulic machines which are connected to the same output shaft. The first hydraulic machine may have a first fixed displacement and the second hydraulic machine may have a second fixed displacement, the second fixed displacement may be the same as or different from the first fixed displacement. By using only fixed displacement hydraulic machines instead of variable displacement hydraulic machines, a more energy efficient power system can be achieved.
Furthermore, by using hydraulic machines having different fixed displacement connected to a common output shaft, the number of discrete torque levels that can be provided is increased.
According to one embodiment of the invention, the first hydraulic machine may be a fixed displacement hydraulic machine and the second hydraulic machine may be a variable displacement hydraulic machine. It may be advantageous to use a variable displacement hydraulic machine in combination with one or more fixed displacement hydraulic machines in order to be able to provide a continuously variable output torque.
In one embodiment of the invention, the differential hydraulic pressure level may be provided by using a first hydraulic accumulator having a first hydraulic pressure level and a second hydraulic accumulator having a second hydraulic pressure level, the first hydraulic pressure level being different from the second hydraulic pressure level. By using hydraulic accumulators for providing hydraulic fluid to the hydraulic machines via valve means, energy may be recuperated from the output shaft, via the hydraulic machines and stored in the accumulator, thereby increasing the energy efficiency of the power system. In embodiments where two or more hydraulic accumulators are used in combination with a variable displacement hydraulic machine, it may be sufficient that only the hydraulic accumulator having the highest pressure is connected to the variable displacement hydraulic machine, and the low pressure side of the variable displacement hydraulic machine may then be connected to the accumulator having the lowest pressure, or to a tank representing the low pressure source. However, it may be advantageous to connect all hydraulic accumulators also to the variable displacement hydraulic machine in order to provide increased flexibility during operation of the power system. A further advantage of using one or more hydraulic accumulators in the power system is that it is possible to downsize the supply unit providing energy to the system, as such a supply unit can be dimensioned to handle the average energy requirement while one or more hydraulic accumulators can provide energy to accommodate peak loads.
According to one embodiment of the invention the hydraulic variator unit may advantageously comprise a primary hydraulic machine and a secondary hydraulic machine, wherein at least one of the primary hydraulic machine and the secondary hydraulic machine is hydraulically connected to the work hydraulic circuit; and wherein at least one of the primary and secondary hydraulic machine comprises: a rotating hydraulic machine arranged to provide a torque via an output shaft; a first valve means for providing a first differential hydraulic pressure level over the hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels, a second valve means for providing a second differential hydraulic pressure level over the hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels, an absolute value of the second differential hydraulic pressure level being different from an absolute value of the first differential hydraulic pressure level; and a control unit configured to control the first valve means and the second valve means such that different discrete levels of torque are provided via the output shaft. It is possible to provide different discrete levels of torque by applying a different differential pressures over a hydraulic machine using discrete pressure levels to achieve the different differential pressures.
In one embodiment of the invention, the rotating hydraulic machine may advantageously have a fixed displacement. Thereby, the output torque is controlled through the different discrete differential pressures which may be applied over the fixed displacement hydraulic machine.
In one embodiment of the invention, the power system may advantageously comprise a first source of hydraulic fluid having a first hydraulic pressure level, a second source of hydraulic fluid having a second hydraulic pressure level different from the first pressure level, and a third source of hydraulic fluid having a third hydraulic pressure level different from the first and the second pressure level; wherein the first valve means and the second valve means use one source of hydraulic fluid in common. Through the use of three different sources of hydraulic fluid having different pressure levels, a plurality of different differential pressures can be applied over the fixed displacement hydraulic machine in order to provide different discrete levels of output torque on the output shaft.
Furthermore a difference in pressure between the second pressure level and the third pressure level is advantageously substantially twice the difference between the first pressure level and the second pressure level. By selecting the difference between the highest and the middle of the pressure levels to be twice the size of the difference between the lowest pressure level and the middle pressure level, an optimal number of equidistant levels of output torque can be provided.
Furthermore, there is provided a working machine comprising a power system according to any of the aforementioned exemplary embodiment.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.